OSK.EXE after minimization, how to use code to display again?

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WinXP

C#

API

In the Enter event of TextBox, the method InputArea_Enter is called to display the soft keyboard.
But if I minimize the soft keyboard, it can't be displayed again.
When the soft keyboard is minimized, how can TextBox display the soft keyboard when entering the soft keyboard.

private static Process _oskProcess = null;
       public void InputArea_Enter()
       {
             if (_oskProcess != null)
               {
                   Process[] pro = Process.GetProcessesByName("osk");
                   if (pro != null && pro.Length > 0)
                   {
                       return;
                   }
               }
               try
               {
                   _oskProcess = Process.Start("OSK.EXE");
               }
               catch (Exception ex)
               {
                   MessageBox.Show(ex.Message);
               }

       }

What I have tried:

I tried to use the following methods:

[DllImport ("user32.dll")]
Public static extern int ShowWindow (int hWnd, int nCmdShow);

But there's nothing to do with it.